Counting of particles, such as cells, is of significant use in medicine and public health. One widely used cytometry system involves optical devices, such as flow cytometers, and tags cells of interest with optical labels (such as fluorescent markers) and interrogates them with light sources such as lasers.
The Coulter principle of impedance cytometry, based on resistive-pulse sensing, is well-established for counting cells non-optically. In its original format, Coulter counting allowed for differentiation of cells by size, to enable counting of individual subsets of a mixed population, such as a white blood cell differential. A second generation of impedance spectroscopy methods builds on the original Coulter principle and interrogates cells across a sweep of alternating current (AC) frequencies.
Microfluidic systems have shown unique promise for studying cell function, cell and tissue engineering, disease diagnosis, blood sample preparation, and drug discovery. Very recently, the use of microfluidics to isolate pure populations of leukocyte subsets from whole blood has attracted significant interest for point-of-care diagnostics. While the principle behind a cell isolation approach can be easily adapted to a wide spectrum of clinical applications, detecting these isolated cells remains a technical challenge to be addressed.